refactor: move html_util into legacy core library

lib/dd3_legacy_core/include/dd3_legacy_core.h

@@ -0,0 +1,4 @@
+#pragma once
+
+// Include this header in legacy Unity tests to force-link dd3_legacy_core.
+void dd3_legacy_core_force_link();

lib/dd3_legacy_core/include/html_util.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <Arduino.h>
+
+String html_escape(const String &input);
+String url_encode_component(const String &input);
+bool sanitize_device_id(const String &input, String &out_device_id);